The first stars and first galaxies formed a few hundred million years after the Big Bang. Their emergence transformed the universe: the first heavy elements changed the gas physics and high energy photons reionized their surroundings. Hence, understanding this early era is at the frontier of modern astrophysics and cosmology. It can be well probed with ancient ultra-faint dwarf galaxies that orbit the Milky Way today. Two ultra-faint dwarf galaxies have been particularly interesting in this regard. Reticulum II is the first known "r-process galaxy". A prolific nucleosynthesis event must have gone off in this system very early on so that subsequent stars formed from gas enriched in large amounts of the very heaviest elements. Calculations for the elemental yield of a neutron star merger match the observed chemical abundances of Reticulum II's stars, thus solving a 60 year old puzzle about the astrophysical site of the rapid (r-) process. Tucana II was recently confirmed to be an extremely metal-poor galaxy ([Fe/H]~-3) with member stars up to 8 half light radii away from the center region. Other systems also contain stars in their outskirts, suggesting that such extended "halos" may not be uncommon among the tinyest dwarf galaxies, possibly being a signature of the very first merger events between galaxies at the earliest times.